1. Field of the Invention
The metal skins of foam panels are commonly painted carbon steel, aluminum, or stainless steel of thickness ranging from 0.018" (0.46 mm) to 0.048" (1.22 mm). A structural foam core is sandwiched between two metal skins to form a composite panel structure through chemical or adhesive bond between the foam core and the metal skins.
2. Description of the Prior Art
Composite metal foam panels have been widely used in building wall and roof structures due to their high strength-to-weight ratio and their high insulation value. The depth of the panel normally ranges from 1 inch (25 mm) to 8 inches (203 mm) depending on load or thermal insulation requirements. The width of the panel normally ranges from 12 inches (305 mm) to 48 inches (1219 mm). The panels are fastened to the building frame members such as horizontal wall girts, vertical mullions, or roof purlins in a side-by-side fashion to form the wall or the roof surface.
In a mass production facility, the pour-in-place process is commonly used in which liquid foam is deposited between the two metal skins and contained in a jig to resist the foaming pressure and to retain the shape of the final product. Freon gas is commonly used as the blowing agent in the foam system. The chemical bond developed between the foam core and the metal skin cannot be uniform throughout the contacting surface. Localized weak bonding areas are normally developed due to skin surface contamination, void formation, or cell distortion due to the rolling of foam cells during the foaming process.
When one of the metal skins is subjected to a high surface temperature, such as due to solar gain in an exterior exposure condition, the thermal expansion of the metal skin and the heat causes stretching of the foam cells near the skin, producing an imbalance of the internal pressure of the freon gas contained in the foam cells. This pressure imbalance causes the freon gas to migrate slowly toward the warmer skin. When the developed freon pressure, acting beneath the metal skin, is adequate to overcome the bond strength, metal skin delamination will happen. This type of skin delamination will happen first at the localized weak bonding areas causing localized skin bulging in these areas. This phenomenon is known as thermal blister in the industry. This thermal blistering problem causes not only a reduction in the structural properties of the panel, but also a serious aesthetic problem.
Drilling a small hole through the metal skin to release the freon gas and rebonding the metal skin to the foam core by injecting adhesive through the hole have been used at the job site to repair the thermal blisters. However, this type of repair work is not only costly, but it is also impossible to restore the panel shape perfectly. Replacing the blistered panels does not only involve significant cost, but also cannot guarantee that no additional thermal blister will be developed in the replaced panels.
Since the localized weak bonding areas are concealed beneath the metal skin, there is no readily available easy method for detecting the weak bonding areas. The only known method in the industry is the thermal blistering test where the metal skin is subjected to sustained heating using heat lamps to see if thermal blisters can be developed at the target surface temperature. Normally, each test requires at least two hours of heat exposure to simulate the solar gain condition. Ideally, the job site thermal blistering problem can be prevented if all panels are subjected to the thermal blistering test before shipping to the job site.
However, it is totally impractical and cost-prohibitive due to the required long testing duration. Therefore, only sampe testing is utilized by the panel manufacturers to prevent the likelihood of massive product failures due to thermal blistering and the risk of some panel blisterings is assumed. The composite metal foam panel manufacturers have faced this thermal blistering problem for years without finding a real solution to the problem. Some panel manufacturers have been forced out of the business due to the thermal blistering problem.